Cards carrying a stripe of magnetic tape encoded with information are widely used for various purposes, i.e., automated commercial transactions such as credit purchase, ATM cards and personal identification for controlled access areas. Conventional magnetic stripes consists of fine particles of a ferromagnetic material suspended in a matrix of an organic polymer which is usually supported by a inert plastic film. The magnetic particles are not free to move within the polymer matrix after formation of the stripe is completed. Information encoding is accomplished by altering the magnetic properties of the entrapped particles in selected areas of the stripe in a predetermined pattern by magnetic domain wall rotation.
Magnetic domain wall rotation (MDWR) is by far the most common technique for encoding magnetic stripes. Virtually all credit and ATM cards are encoded this way. Most access control badges are also encoded by magnetic domain wall rotation. To prepare a stripe for encoding by MDWR, a magnetic field must be applied to the stripe of sufficient strength to rotate the magnetic poles inside of the entrapped magnetic particles so that substantially all particles in the stripe are aligned identically, as illustrated in FIG. 1. In this state the stripe is encodable but not yet encoded. To encode the stripe, in a given region of the stripe the magnetic polarity of substantially all the particles therein, by application of a magnetic field to that region, are reversed in polarity such as to be 180.degree. opposed to the polarity of regions adjacent thereto. These localized areas of aligned but opposed magnetic domains are known as flux reversals. See FIG. 2. The most common type of code used on magnetic stripes is Aiken-Bi-Phase or F/2F encoding which requires two flux reversals for every bit of binary data on the stripe. To write a message, the polarities of the bits are flip-flopped to form the desired message. A "1" is formed by reversing the polarities in two adjacent flux reversals, and a "0" is formed by aligning two flux reversals. See FIG. 3. Today the most widely utilized magnetic card decoders are those designed to sense and read 180.degree. flux reversals within the stripe.
Encoding magnetic stripes by magnetic domain wall rotation (MWDR) is very useful because it can be written and read using quite simple equipment. The main limiting factor is that only low to medium magnetic strength materials can be used as the particles. This property of magnetic strength is known as flux or coercivity. For example, common credit card of today is recorded using magnetic materials with coercivities of about 270-300 Oersteds (the units used to measure coercivity).
The reasons why higher coercivity materials have not been not used are several. One of these problems has been the physical characteristics of the high coercivity magnetic material itself. Another primary reason why higher coercivity magnetic stripe materials are not used for encoding by magnetic domain wall rotation is that the higher coercive force of such materials requires a stronger recording magnetic field to alter its magnetic pole orientations. While a credit card can be altered (erased or garbled) by a common 800-1,000 Oersted household magnet, it is theorized that a magnetic stripe manufactured from magnetic materials with coercivities much higher than 5,000 Oersteds would require a magnetic head constructed of very expensive and exotic materials, such as a superconducting material, to encode or re-encode it. This is stated in theory since no magnetic heads of this type have been constructed. Because of this fact, it is highly unfeasible, if not impossible, to encode magnetic materials whose coercivities exceed 5,000 Oersteds by MDWR.
The relative ease by which a standard credit or ATM card can be encoded, erased or re-encoded due to its low coercivity magnetic particles has given rise to some serious problems. An ever increasing problem of concern is the production of counterfeit cards by the magnetic alteration of the encoded information of an otherwise legitimately issued card.
A need for a tamper resistant magnetic stripe exists in many areas of industry. One is that of the credit and debit card industry. Counterfeiting and fraud are already at alarming levels, and growing. More alarming is that much counterfeiting and altering of credit cards is conducted by professional and well financed counterfeiters. A need for modifications in the existing crime preventative approaches is clearly recognized throughout the credit card industry. This industry alone represents a multi-billion dollar segment of the business community that is demanding improvements in existing data authentication practices.
A second industry sector that would greatly benefit directly from a tamper resistant magnetic stripe is the positive identification and access control community. Due to high labor cost, a trend in today's high-tech society is the desire to have machines, rather than security personnel, verify the validity of ID badges. A media that currently is most commonly used to contain personal data on an access credential is magnetic tape, but for the reasons stated the magnetic tape has very little inherent protection against fraud and counterfeiting. Most companies that sell identification badges include a magnetic stripe on the badge as a standard feature. This is being done because an increasing number of companies that require employees to wear positive ID badges are following the trend to use automated entry control portals to control or restrict access into and out of their facilities. This trend toward automated entry portals and the resultant need for increased data security is also directly applicable to government security issues.
To combat the counterfeiters, several techniques have been attempted over the prior years. Some of these techniques include encrypting the data on the stripe, encoding a security checksum from the magnetic jitter of the data, and overlaying the magnetic stripe with a holographic diffraction pattern. However, for the reasons stated previously, the use of very high coercivity magnetic particles (i.e., greater than about 5,000 Oersteds) has not been considered to be a practical alternative to F/2F encode stripes, since conventional magnetic heads are incapable of producing high enough magnetic fields to rotate the magnet domain walls of such entrapped very high coercivity magnetic particles. To encode a stripe containing very high coercivity magnetic particles would require a physical rotation of the particles themselves, as opposed to a change in the polarity of the magnetic domain of the particle.
Over the years some techniques have been examined by which magnetic stripes can be produced and encoded by magnetic particle rotation within the medium in which the particles are suspended, after the encoding of which the medium is rigidified to prevent subsequent particle rotation and alternation of the encoding. Unfortunately, the particle rotation methods previously investigated by industry suffer from various problems. One problem is that they require the encoding to be performed as the badges or cards are produced.
Such efforts to date directed to the design of a tamper-resistant or tamper-detectable magnetic stripe card have been at least two fold. For cards which would be readable by a F/2F encodement reader, magnetic particles having coercivities of about 3,600 to 4000 Oe have been used in the stripe. At one time this presented greater, although by no means insurmountable, difficulties to one deliberately attempting to alter the legitimately encoded information. Namely an external magnetic field of greater strength was required, such as a high strength electromagnet, to deliberately alter the legitimate encoding. However, to even first legitimately encode such card required greater magnetic field strength by the encoder. Generally such cards were utilized wherein security access concerns were paramount and overrode concerns of cost, economics or convenience. Today, however, since the market is saturated with high field strength encoders (i.e., capable of 3,600 Oersteds), such higher coercivity tapes no longer provide extra security against deliberate tampering. They do, however, still provide a higher level of accidental erasure protection.
Most of the other efforts to produce a secure magnetic stripe such as jitter encoding, oblique encoding, holographic overlays, infrared paint overcoats, etc., depend upon a code reader of special design which is incapable of reading F/2F card encodements. For techniques which have attempted to utilize some form of particle rotation feature as its security device, the security channel of the stripe is security data encoded by particle rotation to create magnetic domains polarities which are 90.degree. out of phase with each other according to a predetermine pattern. Thereafter the medium of the stripe is treated to rigidify it and lock the particles in place. Unless the magnetism of the particles is erased, any attempt to alter the security channel encodement by MDWR is unavailing since the reader for the security channel can only read the 90.degree. phase difference encodement created by the locked position of the particles. Even though the particle positioning within all of the stripe may have the 90.degree. out of phase security pattern, the stripe is otherwise encodeable by MDWR in the regular fashion and this unsecured data is alterable, erasable and tamperable. Security therefor depends upon keying the unsecured MDWR data with the unalterable 90% out of phase security data. In one such methodology a polymer-solvent slurry of particles is encoded and locked in place by removal of the solvent during manufacture of the stripe itself. In another, a photopolymerizable monomer/oligomer slurry of particles is encoded by polymerization of selective areas to lock particles therein in place after which the particles in unpolymerized areas are rotated 90.degree. by exposure to an external magnetic field and thereafter the entirety of the stripe is photopolymerized to rigidify it.
Unfortunately, none of the methodologies for producing tamper proof mag cards is wholly satisfactory. Desirably a tamper resistant "mag" card (or stripe carried thereon) should be readable by a F/2F designed decoder and it should be quickly encodeable apart from and long after the manufacture of the stripe itself. That is, it should be shippable and storable in unencoded but encodeable form. None of the prior methods of particle rotation encodement provide for or permit of this. Also, once encoded with a F/2F readable code, this code should be unalterable by anything substantially less in strength than a magnetic head constructed of exotic materials as discussed earlier.